1. Field of the Invention
The present invention relates to compound semiconductor light emitting devices and manufacturing methods thereof and, more particularly, to a semiconductor laser diode and a light emitting diode capable of emitting blue light.
2. Description of the Background Art
FIG. 17 schematically shows a cross sectional structure of a conventional AlGaN/InGaN/AlGaN type compound semiconductor light emitting device (semiconductor laser, light emitting diode) capable of emitting blue light.
Referring to FIG. 17, the semiconductor light emitting device includes a sapphire (0001) substrate 1, a GaN or AlN buffer layer 2, an n-type GaN layer 3, n-type Al.sub.Z Ga.sub.1-Z N (0.ltoreq.Z.ltoreq.1) lower cladding layer 4, a non-doped or Zn-doped In.sub.Y Ga.sub.1-Y N (0.ltoreq.Y.ltoreq.1) active layer (or referred to as a light emitting layer) 5, a p-type Al.sub.Z Ga.sub.1-Z N (0.ltoreq.Z.ltoreq.1) upper cladding layer 7, and a p-type GaN cap layer 8, stacked on sapphire (0001) substrate 1 in this order. An n-type electrode 10 and a p-type electrode 9 are formed at n-type GaN layer 3 and p-type GaN cap layer 8, respectively.
Such compound semiconductor light emitting device is generally manufactured through the steps below by metal organic chemical vapor deposition method (hereinafter referred to as "MOCVD method").
(1) A surface of sapphire substrate 1 is processed at the temperature of approximately 1050.degree. C.
(2) The substrate temperature is lowered to approximately 510.degree. C. to grow thin GaN or AlN buffer layer 2.
(3) The substrate temperature is elevated to 1020.degree. C. to grow n-type GaN layer 3.
(4) The n-type AlGaN lower cladding layer 4 is grown at the same temperature.
(5) The substrate temperature is lowered to approximately 800.degree. C. to grow non-doped InGaN type active layer (or Zn-doped light emitting layer) 5 to have a thickness of approximately 100-500.ANG..
(6) The substrate temperature is increased to approximately 1020.degree. C. to grow p-type AlGaN upper cladding layer 7.
(7) The p-type GaN cap layer 8 is grown at the same temperature.
(8) After etching, p-type electrode 9 and n-type electrode 10 are formed.
In the steps above, active layer 5 including indium is grown at the temperature of approximately 800.degree. C. because a desired indium ratio cannot be obtained at a growth temperature of 1000.degree. C. or higher since the vapor pressure of indium is relatively high. In addition, AlGaN cladding layer 7 is grown at 1020.degree. C. because the layer cannot have high quality crystal unless it is grown at a temperature of 1000.degree. C. or higher.
Therefore, in the steps (4)-(6) above the light emitting device goes through the growth temperature profile shown in FIG. 16. In FIG. 16, the horizontal axis shows the growth direction of the semiconductor and the vertical axis shows the growth temperature.
However, the conventional method of manufacturing compound semiconductor device described above has a problem that, when the substrate temperature is increased to approximately 1020.degree. C. to grow p-type AlGaN upper cladding layer 7, indium liberates from active layer (light emitting layer) 5 including indium formed at the previous step. Such liberation of indium leads to degradation of the interface between active layer 5 and upper cladding layer 7 and makes it difficult to control the mixed crystal ratio of indium and the thickness of active layer 5.